The present invention relates generally to waveform analysis, a particular application of such waveform analysis being in the field of blood pressure monitoring, particularly as relates to automatic monitoring of systolic blood pressure.
The prior art is replete with devices for measuring systolic pressure of a living subject. An old and simple device is a pressurizeable cuff used in combination with a mercury manometer which reads pressure in the cuff and a stethoscope which is used to listen to Korotkof sounds. In another advanced method of measuring blood pressure, the distance from a blood pressure cuff to the wall of an artery is accurately determined by measuring Doppler shifts of sound waves reflected by the artery. In yet other methods for measuring blood pressure intrusive devices are often inserted directly into blood vessels.
Oscillometric methods of determining systolic pressure are also well known in the art. In such methods, the operator observes the representation of the strength of pulsations of pressure within an artery. This can be done visually, as by watching the extent of bouncing at the top of a mercury column in a mercury manometer which is in pressure communication with the cuff or indirectly as by measuring the occlusion which occurs to a blood vessel in the pinna of the ear as pressure is exerted thereon. These oscillometric methods generally define systolic pressure to be the maximum applied pressure with which threshold oscillations are observed to occur. With a typical mercury manometer and pressurized cuff, this pressure would then be the highest pressure which the operator noted bouncing on the top of the mercury column as the pressure in the cuff was slowly and relatively uniformly reduced. However there are inaccuracies associated with this method for determining threshold oscillations, since the inertia of the mercury column does not allow it to noticeably respond to narrow width pressure pulses.
Each of the aforementioned techniques or devices for measuring systolic pressure exhibit some form of shortcoming such as inaccurate response to narrow width pressure pulses or the requirement for sophisticated and/or expensive measuring equipment.
There is described in U.S. Patent application Ser. No. 578,047, filed May 15, 1975 by Link et al for Apparatus and Process for Determining Systolic Pressure, assigned to the present assignee and incorporated herein by reference, a method and apparatus for automatically and relatively simply obtaining accurate systolic blood pressure measurements, thereby overcoming the shortcomings of the aforementioned devices. That device determines systolic pressure by applying pressure to a living test subject by changing pressure in a pressure cuff attached to the subject adjacent a blood vessel; by measuring at the cuff a quantity proportional to a time dependent fluctuating component representative of the pulsatile pressure within the blood vessel, which quantity is proportional to the amplitude of the pulsatile pressure; by determining the maximum value attained by the quantity as the applied pressure is changed; by storing a representation of the maximum value; by determining when the quantity is substantially equal to about one half of the maximum value for an applied pressure greater than the pressure applied when the maximum value occurs or results; and by reading out the applied pressure corresponding to the quantity being substantially equal to about one half of the maximum value, the readout pressure corresponding to the systolic pressure of the subject. The signal from the pressure cuff comprises a fluctuating quantity proportional to a sum, that sum comprising a time dependent fluctuating component proportional to the amplitude of the pulsatile pressure within the blood vessel, which component has a steeply rising wavefront between end diastole and systole, plus the selectively changeable pressure applied externally adjacent the blood vessel by the cuff.
In U.S. Patent application Ser. No. 754,201 by J. D. Haney and W. Jansen for Systolic Pressure Determining Apparatus and Process Using Integration to Determine Pulse Amplitude, filed Dec. 27, 1976, there is described a systolic blood pressure monitor of the general type described in the aforementioned U.S. patent application Ser. No. 578,047 and being improved in a manner assuring increased accuracy in the determination of systolic blood pressure. The improved monitor does not use a peak-to-peak detector for determining the amplitude of the steeply rising wavefront of the fluctuating component of the signal from the cuff, but instead, differentiates the cuff signal to obtain the time derivative of the fluctuating component, and then integrates a portion of the derivative.
The time derivative signal extends above a zero reference level from the time of end diastole through systolic rise to the systolic peak. Thus, the "above 0" area under the time derivative waveform is representative of the peak-to-peak magnitude (diastolic-to-systolic) of a respective blood pressure pulse. By integrating the "above 0" portion of the time derivative waveform an integral value is obtained which is proportional to the area under the waveform and, accordingly, is representative of the peak-to-peak magnitude of the blood pressure pulse. This integral value is then available for use on a beat-to-beat basis for determining the maximum value attained by the fluctuating component from the cuff and later determining one half of that maximum value in the determination of systolic pressure.
In the apparatus of the aforementioned application U.S. Ser. No. 754,201 by Haney and Jansen, the integration of the time derivative waveform was delimited by the positive-going crossing of the zero reference at the beginning and the negative-going crossing of the zero reference at the end. However, certain artifacts in the cuff signal during the diastolic drop, such as due to patient movement, may result in the time derivative waveform exceeding the zero reference for a brief time other than between end diastole and the systolic peak. Although generally much smaller in magnitude than the "above 0" passage of the signal derivative during systolic rise, this otherwise superfluous "above 0" passage of the artifact time derivative may be included in the determination of an integral value for use in the beat-to-beat determination of that peak-to-peak value representative of one half of the maximum peak-to-peak value, and thereby impair the accuracy of the systolic pressure determination.